Direct thermal imaging is a technique in which a substrate bearing at least one color-forming layer, which is typically initially colorless, is heated by contact with a thermal printing head to form an image. In direct thermal imaging there is no need for ink, toner, or thermal transfer ribbon. Rather, the chemistry required to form an image is present in the imaging member itself. Direct thermal imaging is commonly used to make black-and-white images, and is often employed for the printing of, for example, labels and purchase receipts. There have been described in the prior art numerous attempts to achieve multicolor direct thermal printing. A discussion of various color direct thermal imaging methods is provided in U.S. Pat. No. 6,801,233.
A preferred direct thermal imaging member described in the above-mentioned patent comprises three color-forming layers, each affording one of the subtractive primary colors, and is designed to be printed with a single thermal printing head. The topmost color-forming layer develops color in a relatively short period of time when the surface of the imaging member is heated to a relatively high temperature; the intermediate color-forming layer develops color in an intermediate length of time when the surface of the imaging member is heated to an intermediate temperature; and the lowest color-forming layer develops color in a relatively long period of time when the surface of the imaging member is heated to a relatively low temperature. Separating the color-forming layers are thermally-insulating layers whose thickness, thermal conductivity and heat capacity are selected so that temperatures reached within the color-forming layers may be controlled to provide the desired color by appropriate choices of heating conditions of the surface of the imaging member.
The composition of the thermally-insulating layers is ideally chosen so as neither to compromise the chemistry responsible for formation of color in the color-forming layers nor to degrade the stability of the final image.
Each color-forming layer typically comprises a dye precursor that is colorless in the crystalline form but colored in an amorphous form. Materials such as thermal solvents or developers may be incorporated into the color-forming layer to adjust the temperature at which color is formed or the degree of coloration that is achieved.
During heating of the thermal imaging member as it is printed to form an image, or during prolonged storage of the imaging member before or after an image is formed, it is possible that components initially incorporated within a color-forming layer may migrate from that layer into adjoining layers. Such migration of components may produce problems such as unwanted coloration in unprinted regions or changes in the activation temperature or degree of coloration of a color-forming layer, as is discussed in more detail below. It is also possible that materials in adjoining layers may migrate into a color-forming layer and degrade its performance.
In the prior state of the art it has been necessary to provide barrier layers to impede the migration of components from the color-forming layers into the thermally-insulating layers or from the thermally-insulating layers into the color-forming layers. Such additional barrier layers introduce complexity into the process for manufacturing the thermal imaging member, and may also contribute undesirable physical properties to the final article, as discussed in detail below. The need for additional barrier layers would be obviated by a thermally-insulating layer with improved properties, such that migration of components between the thermally-insulating layer and adjacent layers would be impeded or such that, if such migration were to occur, no objectionable consequence would result.